Currently, plastic bottles such as polyethylene terephthalate (PET) bottles; and metallic bottles such as bottle cans are widely used as containers for beverages such as tea beverages and soft drinks. Plastic labels are often attached to these containers, for indication or for imparting decorativeness and/or functionalities. Of such plastic labels, shrink labels and stretch labels are frequently used, because they have advantages such as good decorativeness, good processability (followability to containers), and large display areas. These labels generally have, on their surfaces, coated or printed layers of inks and other coating compositions, in order to impart decorativeness through characters and designs, and/or to impart functionalities such as scratch resistance and slipperiness.
These coating compositions should have processability suitable for their respective label processing steps such as shrinking process, and cured coating layers therefrom should have such toughness as to avoid deterioration in decorativeness and functionalities due typically to scratches during the course of distribution. In recent years, in the field of shrink films, in particular, demands for improved processabilities such as followability to deformation during shrinking process have been increasingly higher due to increasingly complicated shapes of containers, and the need to provide both high processabilities and high toughness simultaneously has become urgent. In addition, demands have been increasingly growing to impart further functions such as blocking resistance, chemical resistance, thermal stability, and adhesion to base materials.
When these coating compositions are used, for example, as printing inks, gravure printing is generally employed as an application procedure, for excellent print quality (expressivity) of printed matter. Such gravure inks are disadvantageously expensive in cost, because they generally contain large amounts of organic solvents, whereby the solvents evaporated during production processes must be treated, and the treatment requires expensive facilities and high maintenance costs typically for exchange of catalysts. In contrast, water-based inks using no organic solvent suffer from low printing speeds which leads to low productivity, because the water-based inks dry slowly.
To meet these requirements, coating compositions containing epoxy resins as base resins and using substantially no solvent have been developed and improved. Such epoxy resins are relatively brittle, and as a possible solution to this, energy ray-curable compositions further containing an oxetane compound in addition to an epoxy compound for imparting flexibility to the epoxy resin (refer typically to Patent Document 1) are known as coating compositions. In the technique disclosed in this document, only basic properties such as viscosity of the composition and tensile strength of the cured article (cured resin) are considered, but properties in practical use, such as coating processability and followability during shrinking process are never considered. This technique is therefore insufficient to be applied to labels, particularly to shrink labels. In addition, no consideration has been made on functions such as chemical resistance.
In contrast, as solvent-free coating compositions excellent in toughness and adhesion, active energy ray-curable ink compositions containing poly(vinyl acetal)s having ethylenically unsaturated groups (refer typically to Patent Document 2) are known. Photo-curable resin compositions containing a cyclic acetal compound, an epoxy compound, and a photo-induced cationic polymerization initiator (refer typically to Patent Document 3) are also known. These compositions, however, do not sufficiently rapidly cure and have relatively high viscosities, whereby, it is difficult to apply them at higher process speed particularly by gravure printing, thus problems remained in improving the productivity.    [Patent Document 1] Japanese Unexamined Patent Application Publication (JP-A) No. H11-140279    [Patent Document 2] JP-A No. 2004-269690    [Patent Document 3] JP-A No. 2004-35862